The present invention relates generally to load sensing devices for train brakes and, more specifically, to a graduated load sensing system.
Load sensing devices are known in the freight industry as xe2x80x9cempty/load valves.xe2x80x9d These are valves that generally provide 100 percent of the brake signal from the brake control valve to the brake cylinder for loads above a pre-selected load value and reduces the brake signal value by a given percentage as a function of the empty/load valve design for loads below the pre-selected load value. A typical example is found in U.S. Pat. No. 5,211,450 and is available from New York Air Brake Corporation. The typical example described in the ""450 Patent is a Model E/L 60. When the car load is below 20 percent of a loaded car, 60 percent of the value of the brake signal is provided to the brake cylinder. A graph showing the brake ratio versus percentage or truck load is illustrated in FIG. 1. Graph A is for a 60 percent valve at a 20 percent of load switchover. Graph B is a 60 percent valve at a 35 percent of load switchover. The empty truck weight is 42,000 pounds, and the loaded truck weight is 107,000 pounds. This is for a lever ratio of 4.75:1 and a brake cylinder with an 8.5-inch diameter. For the 20 percent of load switchover, the brake ratio varies between approximately 15.5 percent to 22.5 percent brake ratio. The empty/load valve with a 35 percent changeover varies between 12.8 percent brake ratio and 21.5 percent brake ratio. It is desirable to have an empty load or load sensing device with a smaller variation of brake ratio over the range of loads.
The present invention provides a load sensing system for a rail car brake system, including at least two load sensing devices connected in series. The first load sensing device receives a brake signal from the brake control valve and provides a first braking signal for loads above a first value. It provides a second braking signal less than the first braking signal for loads below the first load value. The second load sensing device receives the first and second braking signals from a first load sensing device. It provides a third braking signal to the brake cylinder for loads above the second value and different from the first value and a fourth braking signal less than the third braking signal for loads below the second value. For loads above the first and second load values, the first and third braking signals are equal to the brake signal. For loads above the second value, the third braking signal is equal to the received first or second braking signal.
Thus, for loads above the first and second load values, the full brake signal from the control valve is provided as a braking signal of the cylinder. For loads below both the first and second load values, both load sensing devices provide their smaller signals. For loads between the first and second values, only one of the devices is providing the full brake signal, and the other device provides a smaller signal.
Additional load sensing devices may be connected in series with the first and second load sensing devices between the brake control valve and the brake cylinder. They would all have separate load values at which they switch.
Each of the load sensing devices may have a separate load sensing element, or they may have a common load sensing element. Load sensing devices may be in separate or in common housings. The ratio of the two load sensing devices may be equal or different.
The present invention is also a method of assembling a brake system for a rail vehicle, including selecting first and second load sensing devices having different load values. The first load sensing device is connected to receive brake signals from the control valve and to provide a first braking signal for loads above a first value and a second braking signal less than the first braking signal for loads below the first value. The second load sensing device is connected to receive first and second braking signals from the first sensing device. It also provides a braking signal to the brake cylinder for loads above a second value and a fourth braking signal less than the third braking signal for loads below the second load value.